speed up vips_argument_map()

libvips/include/vips/object.h

@@ -332,7 +332,8 @@ int vips_object_get_argument_priority( VipsObject *object, const char *name );
 		/* We have many props on the arg table ... filter out the \
 		 * ones for this class. \
 		 */ \
-		if( g_object_class_find_property( \
+		if( ARG_INSTANCE && \
+			g_object_class_find_property( \
 			G_OBJECT_CLASS( object_class ), \
 			g_param_spec_get_name( PSPEC ) ) == PSPEC ) {
 

libvips/iofuncs/object.c

@@ -346,7 +346,7 @@ vips_argument_table_destroy( VipsArgumentTable *table )
  * non-%NULL value from @fn.
  */
 void *
-vips_argument_map( VipsObject *object,
+vips_argument_map1( VipsObject *object,
 	VipsArgumentMapFn fn, void *a, void *b )
 {
 	/* Make sure we can't go during the loop. This can happen if eg. we